1. Field of the Invention
The present invention relates to an image forming apparatus, method and developing device for use in image forming apparatus, such as copying machines, facsimile machines, printers, and so forth, and more particularly relates to a developing device capable of obtaining stable image density.
2. Discussion of the Background
In recent years, the use of digital type image forming apparatus, in which an image is formed on the basis of digital image data instead of analog image data has seen an increasing demand. In an image forming operation using digital image data, the density of pixels is not successive, i.e., the density of pixels occurs in a stepwise fashion due to the nature of digital data. Digital image forming apparatus using such digital image data are divided into two types, i.e., one type that expresses the density of pixels using two (binary) levels and another type that expresses the density of pixels using multiple levels.
Applicants have determined that the digital image forming apparatus of the type using two levels to express pixel density have drawbacks associated with developing devices usually found therein. Applicants have further determined that these drawbacks relate to the use of a developer-toner having a predetermined density that does not properly adhere to an image portion on a photoconductor being used as a latent image bearing member. Not only does this toner not properly adhere where it should (i.e., an image portion), there is the further drawback that toner improperly adheres to a part on the latent image bearing member other than the image portion, i.e., a background portion, which results in forming an image not having proper pixel density and having a dirty background. Further, Applicants have determined that the developing performance of the aforementioned developing device changes depending on the environmental conditions found where the developing device is installed and/or after a period of time has elapsed which means that the dot in each pixel forming the image may not be stably formed.
The present invention has been made in view of the above discussed and other problems and addresses the above discussed and other problems. A preferred embodiment of the present invention provides a novel digital image forming apparatus of the type using two levels and a developing device for use in this digital image forming apparatus of type using two levels, as well as a method of developing a latent image capable of obtaining a stable density for the developed image.
According to a preferred embodiment of the present invention, an image forming apparatus includes a latent image bearing member having a photoconductive surface with a latent image including image areas, at least some of the image areas having different image potential values thereon and a developing device for performing a two-level developing operation with a one-component developer including toner particles. The developing device further includes a conveyor member to convey the one-component developer from a supply and to deliver the one-component developer with a predetermined charge to a developing region where a conveyor surface portion of the conveyor member is closely spaced from and opposed to a photoconductive surface portion of the latent image bearing member, a thin layer forming device to form the developer on the developer-bearing member into a uniform thin layer, and a voltage source to apply a developing bias voltage to the conveyor member when the two-level developing operation is performed to move at least some of the one-component developer with a predetermined charge adhering to the conveyor surface portion to the photoconductive surface portion to form saturated amounts of the one-component developer on the image areas of the photoconductive surface portion wherein the saturated amounts do not charge with increases of the image potential above a predetermined threshold value to provide an image having a density determined by the saturated amounts.
In one aspect of the preferred embodiment, an adhering amount of the developer on the conveyor member is about 0.5 mg/cm2.
In another aspect of the preferred embodiment, the developing region includes a developing gap formed to be equal to or less than about 150 xcexcm.
In another aspect of the preferred embodiment, the absolute value of the predetermined charge of the one-component developer adhering to the conveyor surface portion is equal to or less than about 10 xcexcC/g.
In another aspect of the preferred embodiment, substantially all of the one-component developer on the conveyor surface portion is moved during the forming of the saturated amounts.
In another aspect of the preferred embodiment, the developing bias voltage applied to the developer-bearing member is an AC voltage superimposed on a DC voltage with the AC voltage having a peak-to-peak voltage value of 600 to 1200 volts and a frequency of 2 to 6 kHz.
In another aspect of the preferred embodiment, the thin layer forming device protrudes a length of 10 to 15 mm.
In another aspect of the preferred embodiment, the thin layer forming device contacts the developer-bearing member with a contact pressure of from about 10 to about 150 g/cm.
In another aspect of the preferred embodiment, the developer-bearing member is provided with a surface roughness of from about 1 to about 4 xcexcm RZ.
In another aspect of the preferred embodiment, the thin layer forming device forms a uniform thin layer having a height corresponding to 1 to 1.5 times a diameter of the toner particles.
According to still another aspect of the preferred embodiment of the present invention, an absolute value of the predetermined charge of the one-component developer is set equal to or less than about 10 xcexcC/g.